This invention relates to a solid oxide fuel cell assembly, and, more particularly, to a fuel cell stacking assembly that can accommodate the dimensional changes resulting from differences in thermal expansion and/or contraction of the fuel cell assembly components, during operation or fabrication.
A fuel cell is a device in which a first reactant, a fuel such as hydrogen or a hydrocarbon, is electrochemically reacted with a second reactant, an oxidant such as air or oxygen, to produce a DC electrical output. A fuel cell includes an anode, or fuel electrode, which enhances the rate at which electrochemical reactions occur on the fuel side. There is also a cathode, or oxidant electrode, which functions similarly on the oxidant side. In the solid oxide fuel cell, a solid electrolyte, made of dense yttria-stabllized zirconia (YSZ) ceramic separates a porous ceramic anode from a porous ceramic cathode. The anode typically is made of nickelous oxide/YSZ cermet, and the cathode is typically made of doped lanthanum manganite.
In such a fuel cell the fuel flowing to the anode reacts with oxide ions to produce electrons and water, which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and thence to the cathode. The electrolyte is a nonmetallic ceramic that is a nonconductor of electrons, ensuring that the electrons must pass through the external circuit to do useful work. However, the electrolyte permits the oxide ions to pass through from the cathode to the anode.
Each individual electrochemical cell, made of a single anode, a single electrolyte, and a single cathode, generates a relatively small voltage. To achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. The cells are connected in series electrically in the stack. The fuel cell stack includes an electrical interconnect between the cathode and the anode of adjacent cells. The fuel cell assembly also includes ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
In one type of fuel cell assembly adapted for use at high operating temperatures, the entire structure is made of ceramics. While such monolithic fuel cells are useful, ceramics have the inherent material characteristic of low ductility and low toughness. Consequently, they are susceptible to damage by mechanical vibrations and shocks. They are also susceptible to thermal shocks and to thermally induced mechanical stresses due to the different thermal expansion characteristics of the components.
Where the fuel cell is to be used at lower temperatures with a low-temperature ceramic electrolyte, some components of the fuel cell may be made of metals. Metal components are generally less expensive to fabricate than ceramic components and have the advantage of improved ductility and fracture toughness. Metal components are therefore more resistant to mechanical and thermal shock damage than ceramics, offering the potential for construction of more robust fuel cells than possible with all-ceramic fuel cells.
In such a low-temperature fuel cell using metals for at least some components and ceramics for at least some components (all known operable solid oxide fuel cell electrolytes are ceramics), there may be large thermal stresses and strains produced during operation of the fuel cell. Metals as a group have much higher coefficients of thermal expansion than do ceramics. When the metal/ceramic fuel cell is heated and cooled, the dimensions of the metal components change more than the dimensions of the ceramic components, leading to thermal strains within the structure. Unless controlled, the thermal strains produce thermal stresses that can lead to failure of the ceramic components or the seals between the ceramic and metal components.
There is a need for a fuel cell design in which a combination of metallic and ceramic components can be used. Such a fuel cell should be resistant to damage from thermal strains and the resulting thermal stresses that arise due to the differences in the thermal expansion coefficients of the components. The present invention fulfills this need, and further provides related advantages.